1. Field of the Invention
The present invention relates to an etching method for a silicon containing layer of an object having a SiO.sub.2 film a silicon containing layer adjacent thereto. This invention is applied, for example, to a case when a polysilicon layer, which is adjacent to the SiO.sub.2 film in a semiconductor wafer, is etched.
2. Description of the Related Art
A dry etching of polysilicon has been used in for example a MOS transistor. As shown in FIG. 1, a MOS semiconductor wafer is formed of a silicon substrate 1, a SiO.sub.2 gate insulating film 2 having a thickness of 100 to 300 .ANG. formed on the silicon substrate 1, a polysilicon layer 3 having a thickness of about 3000 .ANG. formed on the gate insulating film 2, and a resist film 4 formed on the polysilicon layer 3. In etching the polysilicon layer 3 in such a semiconductor wafer, a mixture of HBr gas and Cl.sub.2 gas or HCl gas is used.
However, as shown in FIG. 2, in a portion where the polysilicon layer 3 is step-like formed, a portion 3a whose thickness is larger than the other portions is provided since a wire layer is further formed on a wire layer 7. As a result, in a case where the portion 3a and a portion 3b having a normal thickness are etched to be removed, the portion 3b is first removed by etching. As a result, as shown in FIG. 3, a residual portion 5 is generated at a portion corresponding to the portion 3a. Then, if the residual portion 5 is completely removed, the gate insulting film 2 is etched and a removal portion 6 shown in FIG. 6 may be formed. In other words, the part of the gate insulating film 2 is undesirably removed.
Since it is necessary to suppress damage of the Si substrate in etching polysilicon in the MOS semiconductor wafer, it is unfavorable that the above removed portion is formed. Therefor, it is required that the polysilicon layer be etched so as to have a high selection ratio to the SiO.sub.2 gate insulating film, that is, (etching speed of polysilicon)/(etching speed of SiO.sub.2). If the selection ratio is sufficiently high, the removal portion 6 can be considerably made small, and etching can be performed with a high accuracy of size.
However, in the conventional polysilicon etching, the selection ratio is about 30 and this is insufficient for the accuracy of size.